To make the dipping sauce, whisk together soy sauce, vinegar, water, sesame oil, sugar, and red pepper flakes in a bowl. Set aside.
In a large mixing bowl, whisk together flour, seltzer, egg, and salt until just combined (some lumps are okay; do not overmix).
Heat oil in a deep fryer or large saucepan to 375F. Dip shrimp in batter and fry until the batter is crispy, about 2 to 3 minutes.
Drain on paper towel and serve with dipping sauce.